This invention relates to cutting elements for use in a rock bit and more specifically to cutting elements having an interlocking feature for interlocking the cutting element cutting table with the cutting element substrate.
A cutting element, such as a shear cutter shown in FIG. 1, typically has a cylindrical cemented tungsten carbide substrate body 10. An ultra hard material cutting table (i.e., layer) 12 is bonded onto the substrate by a sintering process. The cutting table has a planar, typically flat upper surface 14. During the sintering process, cobalt from the tungsten carbide substrate mixes with the ultra hard material forming a polycrystalline structure. A depletion in the amount of cobalt mixing with the ultra hard material may result in a more brittle polycrystalline structure or may even prevent the formation of a polycrystalline structure.
Common problems that plague cutting elements and specifically cutting elements having an ultra hard cutting table, such as polycrystalline diamond (PCD) or polycrystalline cubic boron nitride (PCBN) bonded on a cemented carbide substrate, are chipping, spalling, partial fracturing, cracking or exfoliation of the cutting table. These problems result in the early failure of the cutting table and thus, in a shorter operating life for the cutting element. Typically, these problems may be the result of peak (high magnitude) stresses generated on the ultra hard layer at the region in which the cutting layer makes contact with the earthen formation during drilling. Generally, the cutting elements are inserted into a drag bit body at a rake angle. Consequently, the region of the cutting element that makes contact with the earthen formation includes a portion of the ultra hard material layer upper surface circumferential edge 15. These portions of the layers are subjected to the highest impact loads.
One way to attempt to overcome these problems is to increase the thickness of the ultra hard material. Theoretically, an increase in the ultra hard material layer results in increased cutting element impact and wear resistance. However, an increase in the thickness of the ultra hard material layer may result in delamination of the ultra hard material layer from the substrate. Moreover, as the ultra hard material layer thickness increases, the edges and surfaces of the ultra hard material furthest away from the substrate (e.g., the ultra hard material layer upper surface circumferential edge) are starved for cobalt during the sintering process. Consequently, the strength and ductility of these edges is decreased. Thus, the ultra hard material edges subjected the highest impact loads will be brittle and have lower impact and wear resistance resulting in the early failure of the cutting layer.
Another problem associated with increasing the thickness of the ultra hard material layer is that as the ultra hard material volume increases there is an increase in the residual stresses formed on the ultra hard material due to the thermal coefficient mismatch between the ultra hard material layer and the substrate. The cemented carbide substrate has a higher coefficient of thermal expansion than the ultra hard material. During sintering, both the cemented carbide body and ultra hard material layer are heated to elevated temperatures expanding and forming a bond between the ultra hard material layer and the cemented carbide substrate. The heating causes the substrate to expand more than the ultra hard material. As the ultra hard material layer and substrate cool down, the substrate shrinks more than the ultra hard material because of its higher coefficient of thermal expansion. Consequently, thermally induced tensile stresses are formed on the ultra hard material layer and compressive stresses are formed on the substrate.
Furthermore, an increase in the volume of the ultra hard material also results in the buildup of residual stresses on the ultra hard material layer/substrate interface due to the difference in shrinkage between the ultra hard material and the substrate caused by the consolidation of the ultra hard material and the consolidation of the substrate after sintering. The amount of ultra hard material shrinkage is proportional to the relative volume of the ultra hard material to the substrate. If the difference between the volumes of the ultra hard material and the substrate are significant enough, the shrinkage of the ultra hard material due to consolidation may be great enough to generate tensile residual stresses on the ultra hard material layer and compressive residual stresses on the substrate.
Accordingly, there is a need for a cutting element having an ultra hard material table with improved impact and wear resistance as well as increased cracking, chipping, fracturing, and exfoliating characteristics, and thereby an enhanced operating life.